In recent years, a lighting device for vehicle (for example, lighting device for automobile) in which a light emitting diode (LED) instead of an incandescent light bulb (filament light bulb) is used as a light source is put to practical use.
Since the lighting device in which the light emitting diode is used as the light source has a long life, and can reduce power consumption, it is possible to expect the lighting device in which the light emitting diode is used to replace the incandescent light bulb.
Here, the lighting device for vehicle uses a battery as a power source, however, a voltage which is applied to the lighting device fluctuates.
For example, a standard operating voltage (rated voltage) of a general lighting device for vehicle is approximately 13.5 V, however, due to a decrease in voltage in a battery, an operation of an alternator, an influence on a circuit, or the like, a voltage which is applied to a lighting device fluctuates.
For this reason, in a lighting device for vehicle, an operating voltage range (voltage fluctuation range) is determined. For example, the operating voltage range is from 9 V to 16 V in general, and is from 7 V to 16 V in some cases.
When an applied voltage decreases, an amount of light emission decreases even in a case of an incandescent light bulb, or even in a case of a lighting device in which the light emitting diode is used as a light source.
However, when a voltage largely decreases (becomes low voltage), there is a problem in that the amount of light emission of a lighting device in which the light emitting diode is used as a light source becomes smaller than that of the incandescent light bulb.
In this case, it is possible to suppress a decrease in the amount of light emission of the lighting device in which the light emitting diode is used as the light source using a constant voltage element, or the like.
However, when using the constant voltage element, or the like, there is a problem in that the constant voltage element, or the like, is newly necessary, a calorific value becomes large due to a decrease in circuit efficiency (ratio of applied voltage to light emitting diode with respect to input voltage), or a size of a substrate becomes large.
For this reason, there is a concern that minimization, or cost reduction which is desired in a lighting device for vehicle, may be not obtained.